This invention relates to a surface-mountable thermistor device which may be used for protection against an overcurrent. More particularly, this invention relates to an organic thermistor device comprising a thermistor element made of an organic thermistor material, as well as a method of producing such organic thermistor devices.
Organic PTC (positive temperature coefficient) thermistors made of an organic thermistor material are coming to be used as circuit protection units for suppressing overcurrents. Such organic PTC thermistor devices make use of an organic thermistor material obtained by dispersing carbon or the like in a resin material such as polyethylene to provide a positive temperature characteristic (PTC characteristic). They are generally produced, as shown in FIG. 6, by forming surface electrodes 52a and 52b by pressing a metallic foil of nickel or copper on both upper and lower surfaces of a thermistor body 51 of an organic thermistor material shaped in a planar form and then forming outer electrodes 53a and 53b by plating or sputtering. Alternatively, an organic thermistor device may be formed, as shown in FIG. 7, by using an electrically insulating material 54 such as an insulating resin to cover exposed parts such as the thermistor body 51 and the surface electrodes 52a and 52b, leaving only the outer electrodes 53a and 53b exposed.
An organic thermistor device, as described above, may be surface-mounted, as shown in FIG. 8, by electrically and mechanically connecting the outer electrodes 53a and 53b to wiring electrodes (or "lands") 56 on a printed circuit board 55 by a solder reflowing method through a solder fillet 57.
In the case of a PTC thermistor device for protecting a circuit from an overcurrent situation, its resistance value at normal temperatures is desired to be 0.1.OMEGA. or less such that a voltage drop in the PTC thermistor device during the use of the circuit can be avoided. If the specific resistance, the thickness and the cross-sectional area of the PTC thermistor body 51 are .rho., T and S, respectively, the resistance value of the PTC thermistor device is given by .rho.T/S.
If an organic PTC material is to be used for the PTC thermistor device, the fact at the present time is that it is difficult to make the specific resistance equal to or less than 0.5 .OMEGA.cm if this PTC thermistor material must also have the required electrical characteristics when its resistance value changes suddenly under a high-temperature condition. Accordingly, if it is attempted to use such an organic PTC thermistor material to produce an organic PTC thermistor device with resistance value equal to or less than 0.1.OMEGA. at normal temperatures, the result will be a structure as shown in FIG. 7 having surface electrodes 52a and 52b formed on both upper and lower surfaces of a planar thermistor body 51 made of an organic thermistor material by pressing a metallic foil of nickel or copper.
Even if a PTC thermistor device is produced in a form as shown in FIG. 7 with surface electrodes on both upper and lower surfaces of the thermistor body, the thickness of the thermistor body 51 must be made very small and its cross-sectional area large in order to make its resistance value at normal temperatures equal to or less than 0.1.OMEGA.. With prior art organic PTC thermistor devices, therefore, the dimensions of the thermistor body 51 were, for example, 4.5 mm (length).times.3.2 mm (width).times.0.3 mm (thickness).
Although it is an essential requirement for a PTC thermistor device to reduce the resistance value at normal temperatures, this requirement could be satisfied with the prior art technology only by reducing the thickness of the thermistor body and increasing its cross-sectional area (or its planar area). As a result, the planar dimensions of the product remained large and a large space was required for its surface-mounting. Secondly, a relatively large amount of organic thermistor material will be used for the production and this gives rise to an increased production cost. Thirdly, if the thermistor body is very thin, it is likely to become twisted or bent after being mounted. Fourthly, if a large amount of the organic thermistor material is used between the pair of outer electrodes, the action time of the PTC thermistor device becomes long and there may arise situations where a sufficient protective characteristic against overcurrents cannot be obtained and the circuit element to be protected may break before the PTC thermistor device can act.
An attempt may be made to introduce inner electrodes into the PTC thermistor body by stacking organic PTC sheets with an electrode formed thereon, but the layer-forming process including steps of making thinner organic PTC sheets, forming conductors to serve as inner electrodes and stacking up the sheets one on top of another tends to increase the production cost as a whole. Thus, the price of the product will increase significantly and hence such a method is not a practical solution to the problem.